This invention relates to the regeneration of solutions of microetch cleaning compositions used to remove oxidation, fingerprints, and surface contaminants from copper, copper alloys and other metallic surfaces.
Industrial cleaning compositions used to clean metallic surfaces prior to electroplating, molten metal coating and other manufacturing operations often rely on the removal of a thin layer of metal from the surface being cleaned to assure that all the contaminants are removed from the surface. These types of cleaners are often referred to in the industry as microetch cleaners. The composition of microetch cleaners varies widely depending on what type of metal is to be cleaned and microetched. The cleaning composition must be able to dissolve the metal to be cleaned. For example, a cleaning composition designated to function as a microetch cleaner for aluminum must be able to dissolve a thin layer of the aluminum surface being cleaned. A cleaning composition that will microetch the surface of one metal may not microetch the surface of a different metal. In the example of an aluminum microetch cleaner, the cleaning composition might be a dilute aqueous solution of an alkali hydroxide such as sodium hydroxide. Whereas such a cleaning composition is effective as a microetch cleaner for aluminum, it is totally ineffective as a microetch cleaner for most ferrous alloys such as low carbon steel because it will not dissolve low carbon steel.
In most cases the surface layer of metal dissolved by microetch cleaners is limited to about 1,000 microinches (millionths of an inch) or less. This is one reason that cleaning compositions of this type are called microetch cleaners. The amount of metal dissolved by the microetch cleaner is controlled by choice of the metal dissolving agent used in the cleaning composition, the concentration of the metal dissolving agent, the time the metallic surface is exposed to the cleaning composition, the temperature of the cleaning composition, spray pressure and nozzle configuration (if the cleaning composition is sprayed onto the metallic surface), and other conditions of the cleaning composition and application method of the cleaning composition. Most often the etch rate is about 100 microinches per minute or less.
Although metal etching agents used in microetch cleaners may be the same or similar to agents commonly used to dissolve gross quantities of metals, a clear distinction must be made between solutions designed to dissolve all or substantially all of a metal part and a microetch cleaning process in which 1,000 microinches or less of surface metal is removed in the cleaning process. The objective of a gross metal dissolving solution is to dissolve all or essentially all of the metal exposed to the dissolving solution. The objective of a microetch cleaning composition is to remove surface contaminants and expose a fresh layer of virgin metal on the surface of the part in preparation for subsequent manufacturing operations. Two examples of subsequent manufacturing operations which typically follow microetch cleaning are: electroplating and application of molten metal deposits onto the microetch cleaned surface.
The compositions of microetch cleaners are typically different from gross metal dissolving solutions in the respect that they are lower in concentration of metal dissolving agents than gross metal dissolving solutions.
In order to keep the metal removal rate within the microetch range, fairly low concentrations of metal dissolving agents are used in the cleaning composition. Because microetch cleaning compositions typically have rather low concentrations of metal dissolving agents, they often have a limited useful life. The rather low concentration of metal dissolving agents is often depleted before the other constituents of the cleaning composition are depleted. Thus it is desirable to be able to regenerate or rejuvenate the metal dissolving agent in the cleaning composition. This invention discloses a process for rejuvenation or regeneration of one type of microetch cleaning composition.
Microetch cleaning compositions used to remove fingerprints, oxidation and miscellaneous soils from copper and copper alloys are well known to the industry. The cleaning compositions are usually aqueous solutions containing an oxidizing agent such as ferric ions, cupric ions or dichromate ions; a pH control agent, such as a non-oxidizing acid like sulfuric acid or hydrochloric acid, or an acidic salt of a strong acid and a weak base, such as ferric chloride or cupric chloride; and, optionally, surfactants, water soluble solvents and corrosion inhibitors.
One of the functions performed by the ingredients in the cleaning composition is pH control. The acid or the acidic salt of a strong acid and a weak base dissolve oxidation from the surface of the parts to be cleaned. The oxides of copper and oxides of alloying metals are soluble in many acidic aqueous solutions. The pH control agents keep the aqueous solutions strongly acidic enough to dissolve the oxides from the surface. Little if any of the unoxidized metal underlying the oxide film is dissolved by the acidic solution. After the initial oxide film has been dissolved into the acidic solution, there is no further dissolution of unoxidized metal. Thus, an aqueous solution of a non-oxidizing acid does not function as a microetch.
Another function performed by the ingredients of the microetch cleaning composition is oxidation. The oxidizing agent is believed to oxidize the surface of the copper metal so that the oxides of copper formed by the oxidizing agent can be dissolved by the acidic, aqueous solution of pH control agent. An oxidizing agent alone provides little or no microetching of a copper surface when no pH control agent is present to dissolve the oxidized copper from the surface. The basis metal is microetched by the co-operative action of the oxidizing agent and the acidic aqueous solution of pH control agent. In some cases a single chemical compound can provide both pH control and oxidizing properties. Examples of such compounds are cupric chloride, ferric sulfate, and ceric perchlorate.
Often the oxidizing agent is the higher oxidation state cation of a metal which can exist in more that one cationic oxidation state. In the examples above, ferric (Fe.sup.+3) cupric (Cu.sup.+2), and ceric (Ce.sup.+4) ions are the higher oxidation state cations. Ferrous (Fe.sup.+2), cuprous (Cu.sup.+), and cerous (Ce.sup.+2) are the corresponding lower oxidation state cations.
In some cases the oxidizing agent is the higher oxidation state anion of a complex metal ion which can also exist as a lower oxidation state anion or cation. Examples of this type oxidizing agent are the dichromate anion (Cr.sub.2 O.sub.7.sup.-2) in which the chromium exists in the +6 oxidation state and permangante anion (MnO.sub.4.sup.-1) in which the manganese exists in the +7 oxidation state. Corresponding lower oxidation state ions are chromic cation (Cr.sup.+3), in which the chromium exists in the +3 oxidation state, and the tetravalent manganese cation (Mn.sup.+4) with its +4 oxidation state.
Other ingredients used in microetch cleaning compositions may include surfactants, water soluble solvents and corrosion inhibitors. The presence of these ingredients in microetch cleaning compositions varies widely. Most microetch cleaning compositions contain surfactants to emulsify and lift oily soils off the metallic surfaces to be cleaned. Some cleaning compositions contain water soluble solvents such as lower molecular weight alcohols and/or glycols or glycol ethers. These often function as solvents for oily soils to be removed from the surfaces to be cleaned. Corrosion inhibitors are sometimes included in the cleaning composition. They provide protection for metals which may be exposed to the cleaning composition but which are not to be microetched. They may also provide resistance to tarnishing of the microetch cleaned metal surface after the cleaning process is completed. Examples of such corrosion inhibitors for copper and copper alloys are mercaptobenzothiazole and benzotriazole.
When the cleaning compositions are used to clean and microetch copper surfaces, the oxidizing agent is often depleted well before the other ingredients in the cleaning composition are depleted. It is economically advantageous to regenerate or rejuvenate the oxidizing agent so the performance of the cleaning composition can be restored without replacing the entire cleaning composition. Since many cleaning compositions used in industrial settings are dilute aqueous solutions of concentrated proprietary blends of the active ingredients, one traditional method to restore the performance of the cleaning composition is to simply add more proprietary concentrate to the cleaning composition. This replaces the depleted oxidizing ingredient in the cleaning composition. However, this has the disadvantage of adding more of all the other active ingredients to the cleaning compositions at the same time the replacement oxidizing agent is added. This method is less costly than replacing the entire cleaning composition, but it introduces some excess cost because of the addition of active ingredients that have not been depleted. Furthermore, since the oxidizing agent is often a metal salt which forms a water insoluble hydroxide during precipitation methods of waste treatment of spent solutions of the cleaning compositions, additions of concentrated proprietary blends increases the sludge generated during waste treatment. The spent solutions of the cleaning composition usually contain hazardous heavy metal ions. During precipitation waste treatment, these ions co-precipitate along with metal hydroxides from the oxidizing agent. The hazardous heavy metal sludge and the metal hydroxide sludge from the oxidizing agent are inseparable. Under current environmental regulations, mixtures of non-hazardous metal hydroxide sludge and hazardous heavy metal sludge are classified as hazardous. Thus, the total volume of hazardous waste sludge that must be disposed of increases substantially. The cost of disposal of hazardous sludges increases proportionally.
In other waste treatment techniques, such as ion exchange or electrowinning, the presence of excessive metal salt oxidizing agents contributes excessive costs to the waste treatment of spent microetch cleaning compositions. Excessive metal ions block exchange sites on ion exchange resins. The metal ions from the oxidizing agent attach to exchange sites on the resin where heavy metal ions would normally attach. This results in the need for more frequent regeneration of the ion exchange resin and excessive costs.
Excessive oxidizing agents interfere with electrowinning methods of waste treatment by consumption of excessive electricity during the electroplating operation. This occurs because the oxidizing agent must be reduced before the heavy metals in the spent solution can be electrowon. Additionally some of the oxidizing agents, notably ferric ions, often adversely affect the quality of the electroplated deposit formed in the electrowinning process. Rather than forming a dense, adherent deposit on the cathode of the electrowinning cell, ferric ions often cause the deposit to be loose and porous. This often makes the removal of the electroplated deposit from the electrowinning cell difficult.
Another traditional method to restore the performance of microetch cleaning compositions in which the oxidizing agent has been depleted is to simply add more oxidizing agent to the cleaning composition. Whereas this approach is less costly than adding concentrated proprietary blends as outlined above, the introduction of additional metal ion type oxidizing agent exacerbates all the problems with waste treatment previously discussed.